Polyethylene Molding Composition for Producing Blown Films Having Improved Processability

ABSTRACT

The invention relates to a polyethylene molding composition having a multimodal molar mass distribution particularly suitable for blow molding films having a thickness in the range from 8 to 200 μm. The molding composition has a density at a temperature of 23° C. in the range from 0.953 to 0.960 g/cm 3  and an MFR 190/5  of the final product after extrusion in the range from 0.10 to 0.50 dg/min. The composition comprises from 30 to 60% by weight of a first ethylene polymer fraction made of a homopolymer A having a first molecular weight, from 22 to 40% by weight of a second ethylene polymer fraction made of a further homopolymer or first copolymer B of ethylene and at least one first comonomer from the group of olefins having from 4 to 8 carbon atoms, the first copolymer B having a second molecular weight higher than the first molecular weight, and from 10 to 30% by weight of a third ethylene polymer fraction made of a second copolymer C having a third molecular weight higher than the second molecular weight. The molding composition of the invention allows to produce thin films having improved processability without impairing the mechanical properties.

This application is the U.S. national phase of International ApplicationPCT/EP2006/060223, filed Feb. 23, 2006, claiming priority to GermanPatent Application 102005009896.7 filed Mar. 1, 2005; the disclosures ofInternational Application PCT/EP2006/060223, and German PatentApplication 102005009896.7, each as filed, are incorporated herein byreference.

FIELD OF THE INVENTION

The present invention relates to a polyethylene (PE) molding compositionhaving a multimodal molar mass distribution, i.e. a molding compositioncomprising a plurality of ethylene polymer fractions having distinctmolar masses.

In the present description and in the following claims, unless otherwiseindicated, the term “polymer” is used to indicate both a homopolymer,i.e. a polymer comprising repeating monomeric units derived from equalspecies of monomers, and a copolymer, i.e. a polymer comprisingrepeating monomeric units derived from at least two different species ofmonomers, in which case reference will be made to a binary copolymer, toa terpolymer, etc. depending on the number of different species ofmonomers used.

The multimodal PE molding composition of the invention is particularlyuseful for producing blown films.

The invention also relates to a process for preparing this PE moldingcomposition.

The invention further relates to a blown film produced from theabove-mentioned molding composition by a blown film process.

PRIOR ART

Polyethylene is used on a large scale for producing films by a blownfilm extrusion process thanks to the mechanical strength,processability, good chemical resistance and low intrinsic weight ofpolyethylene.

So, for example, EP-A-0 603 935 describes a molding composition based onpolyethylene which has a bimodal molar mass distribution and is suitablefor producing films and moldings having good mechanical properties.

However, the prior art films made of bimodal polyethylene have aninadequate processability, in particular in terms of bubble stabilityduring processing, and an insufficient drawing capability. Attempts toattain an improved bubble stability inevitably resulted in anunacceptable worsening of the mechanical properties, particularly interms of Dart Drop Impact strength (DDI), which is determined inaccordance with ASTM D 1709, method A.

SUMMARY OF THE INVENTION

The technical problem underlying the present invention is therefore thatof providing a novel PE molding composition having an improvedprocessability in the blown film extrusion process without impairing themechanical strength, particularly in terms of DDI. More in particular,the mechanical strength of films produced from the novel PE moldingcomposition of the invention, expressed as DDI, should not be lower than280 g for a film having a thickness of 20 μm.

For the purpose of the present description and of the claims whichfollow, except where otherwise indicated, all numbers expressingamounts, quantities, percentages, and so forth, are to be understood asbeing modified in all instances by the term “about”. Also, all rangesinclude any combination of the maximum and minimum points disclosed andinclude any intermediate ranges therein, which may or may not bespecifically enumerated herein.

The above-mentioned technical problem is solved by a PE moldingcomposition having a multimodal molar mass distribution, a density at atemperature of 23° C. in the range from 0.953 to 0.960 g/cm³ and aMFR_(190/5) of the final product after extrusion in the range from 0.10to 0.50 dg/min, the polyethylene molding composition comprising:

-   -   from 30 to 60% by weight of a first ethylene polymer fraction        made of an ethylene homopolymer A having a first molecular        weight,    -   from 22 to 40% by weight of a second ethylene polymer fraction        made of a further homopolymer or first copolymer B of ethylene        and at least one first comonomer from the group of olefins        having from 4 to 8 carbon atoms, said first copolymer B having a        second molecular weight higher than said first molecular weight,        and    -   from 10 to 30% by weight of a third ethylene polymer fraction        made of a second copolymer C of ethylene and at least one second        comonomer, said second copolymer C having a third molecular        weight higher than said second molecular weight,    -   all percentages being based on the total weight of the molding        composition.

In the present description and in the following claims, the melt flowrate MFR_(190/5) is the melt flow rate measured in accordance with ISO1133 at 190° C. and under a load of 5 kg. The density is determined inaccordance with ISO1183.

Advantageously, the films produced from the novel PE molding compositionof the invention have a better bubble stability, a reduced melt pressureand adequate mechanical properties when compared to the prior art films,in the sense that the DDI is above 280 g for a film having a thicknessof 20 μm.

The polyethylene molding composition of the invention has a density at atemperature of 23° C. in the range from 0.953 to 0.960 g/cm³, preferablyfrom 0.955 to 0.959 g/cm³, and a broad trimodal molar mass distribution.

According to a preferred embodiment of the invention, the polyethylenemolding composition comprises:

-   -   from 42 to 52% by weight of the first ethylene polymer fraction,        i.e. of the homopolymer A,    -   from 27 to 38% by weight of the second ethylene polymer        fraction, i.e. of a further homopolymer or of the first        copolymer B, and    -   from 15 to 25% by weight of the third ethylene polymer fraction,        i.e. of the second copolymer C.

The second copolymer B preferably contains, in addition to ethylene,predetermined proportions, preferably from 0.1 to 1.0% by weight basedon the weight of the second copolymer B, of at least one first olefincomonomer having from 4 to 8 carbon atoms.

Examples of such comonomer(s) are 1-butene, 1-pentene, 1-hexene,1-octene and 4-methyl-1-pentene and mixture thereof.

In an analogous manner, the second copolymer C is preferably a copolymerof ethylene and of at least one second comonomer preferably selectedfrom the group of olefins having from 4 to 8 carbon atoms, morepreferably from the above-mentioned list of comonomers.

Preferably, the at least one second comonomer is present in an amount offrom 1 to 15% by weight, based on the weight of the second copolymer C.

Furthermore, the PE molding composition of the invention has a melt flowrate MFR_(190/5) of the final product after extrusion in accordance withISO 1133, in the range from 0.10 to 0.50 g/10 min, preferably from 0.32to 0.42 g/10 min.

Preferably, the PE molding composition of the invention has a viscositynumber VN₃, measured in accordance with ISO/R 1191 in decalin at atemperature of 135° C., in the range from 270 to 400 cm³/g, inparticular from 320 to 400 cm³/g.

If, as provided by a preferred embodiment of the invention describedmore in detail in the following, the PE molding composition is preparedby means of a cascaded polymerization process comprising at least threesuccessive polymerization stages comprising a first stage, a secondstage and a third stage, the trimodality of the composition of theinvention can be described in terms of viscosity numbers VN, measured inaccordance with ISO/R 1191, of the ethylene polymer fractions formed inthe different subsequent polymerization stages.

Here, the different viscosity numbers will be indicated as explained inthe following.

The viscosity number VN₁ shall be used to indicate the viscosity numbermeasured on the polymer after the first polymerization stage. Theviscosity number VN₁ is identical to the viscosity number VN_(A) of thehomopolymer A.

According to a preferred embodiment of the invention, the viscositynumber VN₁ is in the range from 60 to 110 cm³/g, more preferably from 60to 110 cm³/g.

The viscosity number VN₂ shall be used to indicate the viscosity numbermeasured on the polymer after the second polymerization stage. Theviscosity number VN₂ is therefore the viscosity number of the mixture ofhomopolymer A plus further homopolymer or first copolymer B. Theviscosity number of the further homopolymer or of the first copolymer Bformed in the second polymerization stage can be instead determined onlymathematically.

According to a preferred embodiment of the invention, the viscositynumber VN₂ is in the range from 250 to 400 cm³/g, preferably from 300 to370 cm³/g.

The viscosity number VN₃ shall be used to indicate the viscosity numbermeasured on the polymer after the third polymerization stage. Theviscosity number VN₃ is therefore the viscosity number of the mixture ofhomopolymer A plus further homopolymer or first copolymer B plus secondcopolymer C. The viscosity number of the second copolymer C formed inthe third polymerization stage can be instead determined onlymathematically.

According to a preferred embodiment of the invention, the viscositynumber VN₃ is in the range from 270 to 400 cm³/g, in particular from 320to 440 cm³/g.

The PE molding composition of the invention may further compriseadditional additives. Such additives may be, for example, heatstabilizers, anti-oxidants, UV stabilizers, light stabilizers, metaldeactivators, peroxide-destroying compounds, basic co-stabilizers inamounts of from 0 to 10% by weight, preferably from 0 to 5% by weight,but also fillers, reinforcing materials, plasticizers, lubricants,emulsifiers, pigments, optical brighteners, flame retardants,antistatics, blowing agents or combinations of these in total amounts offrom 0 to 50% by weight, based on the total weight of the composition.

The present invention also relates to a process for preparing apolyethylene molding composition as described above, comprising the stepof polymerizing ethylene, said at least one first comonomer and said atleast one second comonomer in suspension at a temperature preferably inthe range from 20 to 120° C., more preferably from 70 to 90° C. and,still more preferably, from 80 to 90° C., and at a pressure preferablyin the range from 2 to 10 bar and, preferably, in the presence of aZiegler catalyst.

The process for preparing the PE molding composition is preferablycarried out in the presence of a catalytic system comprising a highlyactive Ziegler catalyst comprising a transition metal compound and aco-catalyst, preferably an organo-aluminum compound, by means of amultistage reaction sequence comprising at least three successivepolymerizations.

Preferably, the polymerization is carried out in multiple successivepolymerization stages comprising a first stage, a second stage, and athird stage performed in corresponding multiple reactors comprising afirst reactor, a second reactor and a third reactor arranged in series.

The polymerization is preferably carried out in a cascaded suspensionpolymerization as described in EP-A-1 228 101.

The molar mass in each polymerization stage is preferably adjusted bymeans of a chain transfer agent, preferably hydrogen, and preferably insuch a manner that the above-mentioned preferred values of viscositynumbers are obtained after each polymerization stage.

The PE molding composition of the invention is particularly suitable forthe production of blown films by the blown film extrusion process. Apossible way to carry out such process is detailed in the following.

The polyethylene molding composition is preferably firstly plasticizedat temperatures in the range from 200 to 250° C. in an extruder.Subsequently, the plasticized polyethylene is extruded in the moltenstate through an annular die so as to form a bubble having asubstantially tubular form. The bubble is cooled, preferably by means ofcompressed air, and subsequently collapsed by means of rollers androlled up into a film.

The molding composition of the invention can be processed particularlywell by the film blowing process because this composition ensures animproved drawing capability and an adequate film bubble stability evenunder the typical processing conditions of large scale industrialplants. In other words, thanks to the drawing capability, particularlythin films having a regular and constant thickness may be produced.

Thanks to the bubble stability, the film bubble coming out from theannular die remains stable even at high take-off speeds and shows notendency to alter its geometry neither in axial direction nor in radialdirection. Preferably, the bubble has a frost line delimiting the moltenmaterial from the solidified material oscillating not more than ±2 cm inaxial direction during the shock test (performed as detailed infollowing Example 3) at a maximal take-off speed.

The invention further relates to a film comprising a PE moldingcomposition as described above and having a thickness in the range from8 to 200 μm, preferably from 10 to 100 μm, more preferably from 8 to 50μm and, still more preferably, from 8 to 10 μm. Preferably, the DDI of afilm having a thickness of 20 μm is higher than 280 g.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will be further described by means of thefollowing preferred embodiments without restricting the scope of theinvention.

EXAMPLE 1 Polymerization (Invention)

Ethylene was polymerized in a continuous process performed in a cascadedmode in three reactors reciprocally arranged in series. A Zieglercatalyst prepared by the method of EP-A 401 776, Example 1, was used,having an extremely high responsiveness to hydrogen and an activitysufficient to carry out the cascaded polymerization, since this catalystwas able to maintain the activity over a long period, from 1 to 8 hours.

The catalyst had in particular the following analytical composition:

Ti  6.2% by weight Mg 70.8% by weight Cl  23.0% by weight.

The catalyst was pre-activated by means of a sufficient amount oftriethylaluminum and then fed into a first reactor in an amount of 4.8mmol/h.

Sufficient suspension medium, in particular hexane, ethylene andhydrogen were additionally fed in the first reactor. The amount ofethylene (=46 kg/h) and the amount of hydrogen (=55 g/h) were set insuch a manner that a percentage of 16.8% by volume of ethylene and apercentage of 68% by volume of hydrogen were measured in the gas space(gas temperature for the analytical measurement=5±1° C.) of the firstreactor. The remainder was a mixture of nitrogen and vaporizedsuspension medium.

The polymerization in the first reactor was carried out at a temperatureof 84° C. and under a pressure of 8.8 bar, corresponding to 0.88 MPa.

The suspension from the first reactor was then conveyed into a secondreactor arranged in series with and downstream of the first reactor. Thepercentage of hydrogen in the gas space (gas temperature for theanalytical measurement=5±1° C.) in the second reactor was reduced to8.6% by volume by means of an intermediate H₂ depressurization. Anamount of 30.7 kg/h of ethylene together with a very small amount of afirst comonomer, namely 1-butene, were introduced into the secondreactor. 62.5% by volume of ethylene, 8.6% by volume of hydrogen and0.4% by volume of 1-butene were measured in the gas space of the secondreactor; the remainder was a mixture of nitrogen and vaporizedsuspension medium.

The polymerization in the second reactor was carried out at atemperature of 84° C. and under a pressure of 2.7 bar, corresponding to0.27 MPa.

The suspension from the second reactor was conveyed via a furtherintermediate depressurization operated without off-gas into a thirdreactor arranged in series with and downstream of the second reactor.The hydrogen concentration was set to 14.6% by volume in the gas spaceby introducing hydrogen. Apart from 19.2 kg/h of ethylene, 240 g/h of asecond comonomer equal to the first comonomer introduced in the secondstage, namely 1-butene, and 6.9 g/h of hydrogen were additionallyintroduced into the third reactor.

A percentage of ethylene of 66% by volume, a percentage of hydrogen of14.6% by volume and a percentage of 1-butene of 1% by volume weremeasured in the gas space of the third reactor (gas temperature for theanalytical measurement=5±1° C.); the remainder was a mixture of nitrogenand vaporized suspension medium.

The polymerization in the third reactor was carried out at a temperatureof 84° C. and under a pressure of 3 bar, corresponding to 0.3 MPa.

The suspension medium was separated off from the polymer suspensionleaving the third reactor and the powder was dried and passed topelletization.

The polyethylene molding composition prepared as described above had adensity of 0.957 g/cm³, viscosity numbers VN₁, VN₂ and VN₃, proportionsW_(A), W_(B) and W_(C) of the homopolymer A, of the first copolymer Band, respectively, of the second copolymer C and melt flow rates MFR₁,MFR₂ and MFR₃ which are reported in Table 1 below.

TABLE 1 Example 1 w_(A) [% by weight] 48 w_(B) [% by weight] 32 w_(C) [%by weight] 20 VN₁ [cm³/g] 81 VN₂ [cm³/g] 337 VN₃ [cm³/g] 365MFR_(1(190° C./1.2 kg)) [g/10 min] 85 MFR_(2(190° C./5 kg))[g/10 min]1.1 MFR_(3(190° C./5 kg)) [g/10 min] 0.65 MFR_(pellets(190° C./5 kg))[g/10 min] 0.39

The abbreviations for the physical properties in Table 1 have thefollowing meanings:

-   -   W_(A), W_(B), W_(C)=proportion of homopolymer A, first copolymer        B and, respectively, second copolymer C in the total molding        composition=reactor split, determined by the amount of ethylene        fed into the respective reactor;    -   VN₁, VN₂, VN₃=viscosity number of the polymer leaving the first,        second and, respectively, third reactor measured in accordance        with ISO/R 1191 in decalin at a temperature of 135° C.;    -   MFR₁, MFR₂, MFR₃=melt flow rate of the polymer leaving the        first, second and, respectively, third reactor, measured in        accordance with ISO 1133 with indication of the temperature and        the load;    -   MFR_(pellets)=melt flow rate of the final product after        extrusion.

EXAMPLE 2 Film Preparation (Invention)

From the molding composition so prepared, a film was produced in thefollowing way.

A film having a thickness of 20 μm was produced on an Alpine filmblowing plant comprising an extruder with a diameter d₁ of 50 mm and alength of 21×d₁(=1.05 m) and an annular die having a diameter d₂ of 120mm and a gap width of 1 mm. The film was produced at a blow-up ratio of4:1 and a neck length of 7.5×d₂(=90 cm). The melt temperature of themolding composition in the extruder was 205-210° C.

The film properties are shown in Table 2 below.

EXAMPLE 3 Film Preparation (Comparison)

A 20 μm film was produced using a commercial film raw material fromBorealis, which is commercially available under the designation FS 1560,on the same plant and under the same conditions described in Example 2with the exception that the melt temperature of the molding compositionin the extruder was 205-215° C.

The film properties are shown in Table 2 below.

TABLE 2 Example 2 (invention) Example 3 (comparison) Take-off: 58m/min + + Shock test: + + Take-off: 63 m/min + + Shock test: + −Take-off: 70 m/min + − Shock test: + − Take-off: 77 m/min + − Shocktest: + − Take-off: 87 m/min + − Shock test: + − DDI [g] 290 310 SpecksNo specks high specks count Melt pressure [bar] 330 340

More in particular, the film bubble stability was determined by thefollowing procedure, including a preliminary test and a shock test asdetailed below.

In the preliminary test, the take-off speed was set at predeterminedincreasing take-off speeds, namely ar 58, 63, 70, 77 and 87m/min(=maximum rolling-up speed). After the respective take-off speedhad been attained and the neck length had been adjusted to 90 cm byadjusting the cooling air blower, the axial oscillation of the filmbubble was observed.

The test was considered finished and passed at a given speed if theaxial oscillation of the bubble being formed was in the range of ±2 cmover a period of observation of one (1) minute.

The shock test was subsequently carried out at the same take-off speedsetting as in the preliminary test.

In the shock test, the bubble was made axially oscillate. This wasperformed by fully opening the iris of the cooling air blower for aperiod of about 7 s. The iris was then reset to the initial position.The opening and closing of the iris was monitored via the pressure ofthe cooling air. At room temperature greater than 25° C., however, theopening of the above-mentioned iris alone is not sufficient to set thefilm bubble into oscillation. Accordingly, at temperatures greater than25° C., the iris was firstly opened and then shut completely for amaximum of 3 s, after which it was reset to the initial position, alwaysmonitoring by means of the air pressure. The shock test was consideredpassed at a given take-off speed if the oscillations of the film bubblehad abated to ±2 cm within 2 minutes.

This was made for each one of the above-mentioned increasing take-offspeeds. If the shock test or the preliminary test was not passed at aparticular take-off speed, the stability grade corresponding to theprevious lower take-off speed was awarded.

The dart drop impact strength of the films was determined according tothe standard ASTM D 1709, method A.

The assessment of specks was carried out visually.

1. A polyethylene molding composition having a multimodal molar massdistribution; a density at a temperature of 23° C. measured inaccordance with ISO 1183, in the range from 0.953 to 0.960 g/cm³; and anMFR_(190/5), measured in accordance with ISO 1133, of the final productafter extrusion in the range from 0.10 to 0.50 dg/min, said polyethylenemolding composition comprising: from 30 to 60% by weight of a firstethylene polymer fraction made of an ethylene homopolymer A having afirst molecular weight; from 22 to 40% by weight of a second ethylenepolymer fraction made of a further homopolymer or first copolymer B ofethylene and at least one first comonomer selected from the group ofolefins having from 4 to 8 carbon atoms, said first copolymer B having asecond molecular weight higher than said first molecular weight of thehomopolymer A; and from 10 to 30% by weight of a third ethylene polymerfraction made of a second copolymer C of ethylene and at least onesecond comonomer, said second copolymer C having a third molecularweight higher than said second molecular weight of the copolymer B, allpercentages being based on the total weight of the molding composition.2. The polyethylene molding composition according to claim 1,comprising: from 42 to 52% by weight of the first ethylene polymerfraction; from 27 to 38% by weight of the second ethylene polymerfraction, the first copolymer B containing from 0.1 to 1.0% by weight,based on the weight of copolymer B, of said at least one firstcomonomer; and from 15 to 25% by weight of the third ethylene polymerfraction, the second copolymer C containing from 1 to 15% by weight,based on the weight of the second copolymer C, of said at least onesecond comonomer.
 3. The polyethylene molding composition according toclaim 1, wherein said first comonomer and said second comonomer areindependently selected from 1-butene, 1-pentene, 1-hexene, 1-octene,4-methyl-1-pentene and mixtures thereof.
 4. The polyethylene moldingcomposition according to claim 1, wherein the density is from 0.955 to0.959 g/cm³ and the MFR_(190/5) of the final product after extrusion isin the range from 0.32 to 0.42 g/10 min.
 5. The polyethylene moldingcomposition according to claim 4, further comprising a viscosity numberVN₃, measured in accordance with ISO/R 1191 in decalin at a temperatureof 135° C., in the range from 270 to 400 cm³/g.
 6. A process forpreparing a polyethylene molding composition comprising from 30 to 60%by weight of a first ethylene polymer fraction made of an ethylenehomopolymer A having a first molecular weight; from 22 to 40% by weightof a second ethylene polymer fraction made of a further homopolymer orfirst copolymer B of ethylene and at least one first comonomer selectedfrom the group of olefins having from 4 to 8 carbon atoms, said firstcopolymer B having a second molecular weight higher than said firstmolecular weight of the homopolymer A; and from 10 to 30% by weight of athird ethylene polymer fraction made of a second copolymer C of ethyleneand at least one second comonomer, said second copolymer C having athird molecular weight higher than said second molecular weight of thecopolymer B. all percentages being based on the total weight of themolding composition, wherein the polyethylene molding composition has amultimodal molar mass distribution; a density at a temperature of 23°C., measured in accordance with ISO 1183, in the range from 0.953 to0.960 g/cm³; and an MFR_(190/5), measured in accordance with ISO 1133,of the final product after extrusion in the range from 0.10 to 0.50dg/min, the process comprising the step of polymerizing ethylene, saidat least one first comonomer and said at least one second comonomer insuspension at temperatures in the range from 20 to 120° C., at apressure in the range from 2 to 10 bar and in the presence of a Zieglercatalyst comprising a transition metal compound and an organo-aluminumcompound.
 7. The process according to claim 6, wherein saidpolymerization step is carried out in multiple successive polymerizationstages comprising a first stage, a second stage, and a third stageperformed in corresponding multiple reactors comprising a first reactor,a second reactor and a third reactor arranged in series, wherein thefirst, second and third stages each comprise a molar mass of apolyethylene composition and a hydrogen concentration, wherein the molarmass of the polyethylene composition prepared in each stage is adjustedin each case by means of hydrogen.
 8. The process according to claim 7,wherein the hydrogen concentration in the first polymerization stage isadjusted to obtain in the homopolymer A a viscosity number VN₁, measuredin accordance with ISO/R 1191 in the range from 70 to 110 cm³/g.
 9. Theprocess according to claim 7, wherein the hydrogen concentration in thesecond polymerization stage is adjusted to obtain in a mixture ofhomopolymer A plus homopolymer or copolymer B a viscosity number VN₂,measured in accordance with ISO/R 1191, in the range from 250 to 400cm³/g.
 10. The process according to claim 7, wherein the hydrogenconcentration in the third polymerization stage is adjusted to obtain ina mixture of homopolymer A plus first homopolymer or copolymer B plussecond copolymer C a viscosity number VN₃, measured in accordance withISO/R 1191, in the range from 280 to 400 cm³/g.
 11. A process comprisingforming a blown film having a thickness in the range from 8 to 200 μm,the film comprising a polyethylene molding composition having amultimodal molar mass distribution; a density at a temperature of 23°C., measured in accordance with ISO 1183, in the range from 0.953 to0.960 g/cm³; and an MFR_(190/5), measured in accordance with ISO 1133,of the final product after extrusion in the range from 0.10 to 0.50dg/min said polyethylene molding composition comprising: from 30 to 60%by weight of a first ethylene polymer fraction made of an ethylenehomopolymer A having a first molecular weight; from 22 to 40% by weightof a second ethylene polymer fraction made of a further homopolymer orfirst copolymer B of ethylene and at least one first comonomer selectedfrom the group of olefins having from 4 to 8 carbon atoms, said firstcopolymer B having a second molecular weight higher than said firstmolecular weight of the homopolymer A; and from 10 to 30% by weight of athird ethylene polymer fraction made of a second copolymer C of ethyleneand at least one second comonomer, said second copolymer C having athird molecular weight higher than said second molecular weight of thecopolymer B, all percentages being based on the total weight of themolding composition.
 12. The process according to claim 11, wherein theblown film is formed via a blown film process comprising the step ofmelting the polyethylene molding composition to obtain a polyethylenemelt, extruding the polyethylene melt by forcing the polyethylene meltthrough an annular die to form a bubble having a frost line oscillatingat a maximum of ±2 cm in axial direction during the shock test at amaximal take-off speed.
 13. A blown film having a thickness in the rangefrom 8 to 200 μm; a dart drop impact DDI, measured in accordance withASTM D 1709 method A, of more than 280 g, measured on a film having athickness of 20 μm, comprising a polyethylene molding composition havinga multimodal molar mass distribution; a density at a temperature of 23°C. measured in accordance with ISO 1183, in the range from 0.953 to0.960 g/cm³; and an MFR_(190/5), measured in accordance with ISO 1133,of the final product after extrusion in the range from 0.10 to 0.50dg/min, said polyethylene molding composition comprising: from 30 to 60%by weight of a first ethylene polymer fraction made of an ethylenehomopolymer A having a first molecular weight; from 22 to 40% by weightof a second ethylene polymer fraction made of a further homopolymer orfirst copolymer B of ethylene and at least one first comonomer selectedfrom the group of olefins having from 4 to 8 carbon atoms, said firstcopolymer B having a second molecular weight higher than said firstmolecular weight of the homopolymer A; and from 10 to 30% by weight of athird ethylene polymer fraction made of a second copolymer C of ethyleneand at least one second comonomer, said second copolymer C having athird molecular weight higher than said second molecular weight of thecopolymer B, all percentages being based on the total weight of themolding composition.